The present invention relates to an ultrasonic transducer used in ultrasonic imaging apparatus for medical diagnostics.
Generally, a phased-array ultrasonic transducer is constructed as shown in FIG. 4. In FIG. 4, numeral 1 designates a backing member shaped like a rectangular prism, and numeral 2 denotes a plurality of ultrasonic transducer elements arranged in an array. Metal electrode layers 2a and 2b are formed on the signal transmission surface side and signal supply surface side, respectively, of the transducer elements 2. Numeral 3 designates a signal electrode plate, one side edge of which is electrically connected to one side edge of the electrode layer 2b, and the other edge portion of which extends from one lateral face of the backing member 1 to the top side thereof. A plurality of signal pins 4 are arranged on the top-side portion of the signal electrode plate 3 so as to be connected to their corresponding transducer elements 2. Numeral 5 designates an acoustic convex lens provided on the transmission surface side of the ultrasonic transducer elements 2. Numeral 6 designates an earth electrode plate which extends over the other lateral face of the backing member 1, located on the earth side of the transducer elements 2. Connected to the metal electrode layer 2a, the earth electrode plate 6 serves as a common earth plate for the ultrasonic transducer elements 2. The signal pins 4 are connected individually to signal wires 7, which are shielded and electrically insulated by a shield member 8. The signal wires 7 are bound into a single bundle 9 at their shielded portions and connected to a transmitter-receiver circuit 10. The shield member of the bundle 9 is connected by soldering or the like to one end of a single and common grounding conductor 1 the other end of which is connected to the earth electrode plate 6.
FIG. 5 shows an equivalent circuit of the prior art ultrasonic transducer, in which a closed circuit is formed for each of transducer elements 2.sub.1 to 2.sub.n. For example, a driving pulser 10.sub.A is connected to one end of the transducer element 2.sub.1, while a receiver circuit 10B is connected to each one end of the other transducer elements 2.sub.n. All the other ends of the transducer elements are grounded in common.
In the ultrasonic probe constructed in this manner, electrical pulses delivered from the transmitter-receiver circuit 10 are supplied to the transducer element 2.sub.1 through their corresponding signal line 7 for ultrasonic vibration, and are also transferred through the grounding conductor l and the earth electrode plate 6 to the grounding-conductor side of the shield members. Each of the shield members is connected to the single and common grounding conductor l, so that part of the electrical pulse transferred from an operating transducer element is supplied to the other shield members 8 in addition to the shield member 8 for the wire that corresponds to the transducer element 2.sub.1 in operation. The above arrangement, in which the grounding terminals of the transducer elements are connected in common to the earth electrode plate 6 and the grounding conductor l is connected to the earth electrode plate 6, by itself has some impedance. This impedance does not permit the electrical signal fed, for example, to the transducer element 2.sub.1 by the driving pulser 10.sub.A to be transmitted to the grounding conductor l in its entirety. Part of it is sent, via the electrode plate 6 to which the transducer elements are connected in common, to the adjacent transducer elements that are not intended to be in operation. Thus, the transducer elements that should not be in operation may operate erroneously. Furthermore, the ultrasonic transducer will transmit unnecessary pulses and receive unnecessary signals produced by transducer elements other than the transducer element in operation, resulting in artifacts in ultrasonic images and failing to provide accurate information on the region to be diagnosed.